We will study specimens of Equisetum.
It is the only extant genus in this Division. Related plants once formed substantial
populations, and some were tree-like. However, during the course of
evolution these plants disappeared from the landscape but have been preserved as fossils.
Equisetum grows in moist
environments. However, it has many adaptations for survival
in dry habitats. The typical plant has a subterranean stem called a Rhizome.
The rhizome produces adventitious roots at each node. Aerial
stems have preformed root apices at each node,
as well. These are activated when part of the stem falls down or when part of the stem
becomes detached. The stems are jointed. This is like having one straw on
top of another & another. Branch primordia are also
present at each node. In some species these produce branches as part of
normal growth. However, there are species which will not branch unless part of the stem is
dislodged from the main stem. There is a weak point at the base of each
internode because Equisetum has an Intercalary
Meristem. The dividing cells are at the node. These have thin
walls and will pop apart if the stem is stretched. In addition,
the stems are hollow, except at the nodes where there is a solid septum.
Thus, each internode-node complex can float because it is light and hollow, and can form
roots and shoots. This is a good strategy for plants that live near water wherein asexual
propagules can float away to unoccupied habitats where they can put down roots and puns.
Why might Intercalary Growth be a hindrance to growing vertically and Standing Tall?
The Roots have a simple anatomy. An Apical Cell produces the Root Cap and Root Body. The stele is centrally located. An Endodermis is present as are a cortex and epidermis.
An Apical Cell at the opposite end of
the plant produces the leaves and shoot. The Leaves are thin and minute
with one Leaf Trace per Microphyll. The microphylls
are fused at each node and form a sheath which is tightly pressed to the
epidermis of the stem. They are not important for photosynthesis. 
The stems are grooved like Psilotum, however, there are more groves present with Equisetum. Stomata are located in the low points of each groove. This is a xeromorphic trait (a trait which has adaptive value for dry habitats). Why is this important?
The Epidermis has a thick,
rough cuticle. The cell walls are impregnated with Silicon which
contributes to its rough texture and brittleness. Thick walled Sclerenchyma
Fibers are present in the subepidermal region opposite each ridge.
These have lots of Silicon in their walls but are NOT Lignified.
Photosynthetic Chlorenchyma occupies the subepidermal regions in the valleys
between the ridges. Could this location have some significance? There is also a canal
in this area. It is called the - However, we can call it the Cortical
Canal since it is in the Cortex. The Vascular Bundles are
directly opposite the surface ridges. These are collateral.
This means that they have xylem & phloem which remain more or
less opposite one another. The xylem in Equisetum
surrounds the Phloem on three sides, however. The arrangement
of vascular bundles in one circle is called a Eustele, and is similar to
Angiosperm Eustele. However an Endodermis is present in the stem.
It may surround each bundle or may skirt each side of the bundle. Angiosperms do not have
anything like this except with a few species. There is a canal in each vascular
bundle. This is caused by the rupture of primary xylem cells during elongation. This is
the carinal or vascular or xylem canal. The
large hollow space in the center of the stem is called the Central Canal.
The Sporangia are
attached to Sporangiophores. The Sporangiophores are assembled into Cones
or Strobili. Equisetum is Homosporous
and the spores are green. They germinate to produce a protonema
which eventually forms a three dimensional, photosynthetic gametophyte. Antheridia
develop first on slightly elevated lobes of the
gametophyte. 
The archegonia are produced later in the vicinity of the basal Rhizoids. Fertilization occurs when flagellated sperm swim to the egg. Embryo development is immediate and Exoscopic. The embryo contains Root and Shoot Apical Meristems and a Foot.
Observe living
specimen if available.
Make note of the nodes and internodes as well as the origin of Roots. Make sure you identify the leaves.
Observe cross sections of Equisetem stems prepared by your lab instructors.
Examine the DEMO of Equisetum Apical Cell.
Study Commercial
Slides of Equisetum Stems. 
This includes
Rhizome
Sterile Stem(Produces only Photosynthetic structures)
Fertile Stem (Produces
a terminal Sporangium)
Locate the Vascular Bundles.
How are they organized?
Note the presence of Canals. How were these created?
Study Commercial
Slides of Equisetum Strobili. 
Look at a long section as well as a cross section.
Note the arrangement
of the
Vascular Bundles in the center of the Strobilus.
How does this compare
with the stem?
Note the presence or absence of canals.
Why is this different from the stem?
The Gametophyte
Observe Gametophyte Whole Mounts
Locate Antheridia
and Archegonia.
of a Gametophyte.
